Bipolar transistors are commonly used in semiconductor devices, especially for high-speed operation and large drive current and low 1/f noise applications. Largely because of these features, heterojunction bipolar transistors (HBTs) are used in products such as integrated switching devices and microwave devices, e.g., in wireless communications, satellite direct broadcast systems, automobile collision avoidance systems, global positioning systems, and other high-frequency applications. Heterojunction bipolar transistors (HBTs) theoretically provide advantages over conventional homojunction bipolar transistors by providing a heterojunction between a base and emitter of a transistor. A heterojunction is formed between two dissimilar semiconductor materials and there will be a bandgap discontinuity between these two materials. A Silicon (Si) homojunction has no bandgap discontinuity at the junction. From the perspective of an NPN transistor, discontinuity in the valence band at the emitter base junction restricts hole flow from the base to the emitter, thus improving emitter injection efficiency and current gain. A discontinuity at the collector base junction outside the space charge region (SCR) can lead to charge pile-up and produce an undesirable buildup of minority carriers in the base. To the extent that injection efficiency and current gain improvements can be achieved, base region resistivity may be lowered (which lowers the base resistance) and emitter region resistivity may be raised (which lowers base-emitter junction capacitance) to create fast transistors without significantly compromising other device parameters. Such fast transistors would be useful for high speed digital, microwave and other integrated circuit and discrete transistor applications.
In practice, HBT performance often falls far short of the theoretical expectations. One conventional Si-based HBT reduces the bandgap of the base region by creating a base material having a narrower bandgap than Si. In particular, a small amount of germanium (Ge) is mixed with Si in the base (Si—xGex), and the emitter is more purely Si. Unfortunately, the amount of bandgap difference (.DELTA.Eg) for as much as 20% Ge content in the base is only about 0.15 eV. This small .DELTA.Eg achieves only a small portion of the performance benefits that HBTs theoretically promise.
Slight improvements in HBT performance have been achieved by using materials other than Si for the emitter of an HBT. Three emitter materials which have been investigated for use in HBT transistors are silicon carbide (SiC), which has a bandgap of 2.93 eV, gallium arsenide (GaAs) which has a bandgap of 1.42 eV, and gallium phosphide (GaP), which has a bandgap of 2.24 eV. Unfortunately, such materials have lattice constants which differ from Si. For example, SiC has a 20% lattice mismatch, GaAs has a 4% lattice mismatch, and GaP has a 0.34% lattice mismatch. Likewise, such materials have thermal expansion coefficients which differ from Si. Si has a thermal expansion coefficient of around 2.6.times.10.sup.−6 (.degree. C.)−1, while GaAs has a thermal expansion coefficient of around 6.7.times.10.sup.−6 (.degree. C.)−1, and GaP has a thermal expansion coefficient of around 5.91.times.10.sup.−6 (.degree. C.)−1. Because of these differences, only thin layers of these materials have been successfully grown on Si without the formation of significant defects. The maximum thickness for a low defect layer of SiC grown on Si is only a few angstroms (.ANG.) and for GaAs grown on Si is less than 200 ANG. At these thicknesses or less, strain which is caused by lattice mismatch is contained by lattice stretching rather than crystal defects. Thinner, low-defect thicknesses of these materials do not possess a sufficient thickness to protect the base-emitter junction from shorting due to diffusion of metal from the emitter contact region. Thicker, high-defect thicknesses of these materials exhibit degraded junction performance due to an excessive number of defects.
The most successful HBT improvements to date are believed to have been achieved by forming a GaP layer over Si at the base-emitter junction. GaP is desirable because it has a relative large bandgap (i.e. about 2.24 eV) and little lattice mismatch with silicon (i.e. about 0.34%). Nevertheless, such conventional HBTs that use a GaP layer over Si still achieve only a small portion of the performance benefits that HBTs theoretically promise. The reason for this poor performance appears to be that a Si—GaP junction suffers from an unusually large amount of interdiffusion, where the Ga and P readily diffuse into the Si, and vice-versa. The interdiffusion between Si and GaP results in a poor semiconductor junction, with the metallurgical junction being displaced from the electrical junction. Accordingly, the performance gains that are suggested by the wide bandgap difference between a Si base and a GaP emitter are not achieved in practice because the resulting diffuse junction negates those potential gains.
In the field of photoelectric semiconductors, it is desirable to form compound structures using a Si substrate and direct gap semiconductor materials. A Si substrate is desirable for mechanical stability and because a manufacturing infrastructure exists for reliably mass producing rugged Si wafers at relatively low cost. The Si substrate is typically an extrinsic part of the photoelectric semiconductor not used in forming intrinsic photoelectric semiconductor junctions.
In an IEEE article entitled “Si/SiGe Epitaxial-Base Transistors—Part II: Process Integration and Analog Applications”, written by D. L. Harame, J. H. Comfort, J. D. Cressler, E. F. Crabbe, J. Y.-C. Sun, B. S. Meyerson and T. Tice, published in 1995, disclosed are conventional techniques for manufacturing super self-aligned transistors.
Another prior art technique is disclosed in U.S. Pat. No. 5,962,879 to Ryum et al. This Patent employs methods that are theoretically simple yet very difficult to perform in practice. FIGS. 4C and 4D show the plurality of layers that must be finely etched in order to create the transistor. It is also noted that the base region of this transistor is quite large and therefore unwanted diffusion and capacitance problems result. These unwanted effects will decrease the transistors power gain and frequency response. The process steps described in U.S. Pat. No. 5,962,879 also allows silicide material to come in direct contact with the active base region and the removal of this material in the active base regions (necessary to obtain a functional npn) is quite difficult without adding additional processing defects.
Therefore, there is a need for a method for forming a super self-aligned heterojunction bipolar transistor that still maintains favorable electrical characteristics.